The present invention relates to the radiographic arts. It finds particular application in conjunction with x-ray tubes for use in computerized tomographic scanners and will be described with particular reference thereto. It is to be appreciated, however, that the present invention will also find utility in other applications, particularly those which are sensitive to even small variations in x-ray tube output.
In CT scanners, like other radiographic equipment, the product of the x-ray flux or output and the exposure time determines the dose of radiation passing through the patient. To increase the speed of CT scanners, hence reduce the time which the x-ray beam irradiates or exposes each radiation detector before sampling, progressively more powerful x-ray tubes have been developed. As this exposure time becomes shorter, the sampled data becomes more sensitive to fluctuations in the x-ray output of the tube.
Commonly, high powered tube housings have a cylindrical central portion which houses a rotating anode x-ray tube and defines an x-ray output window. At opposite ends of the central cylinder, the x-ray housings have enlarged portions or horns. The anode lead wire is connected within one horn and the cathode lead is connected within the other horn.
To electrically insulate the tube and its associated electrical connectors and to remove the large amounts of heat generated by these x-ray tubes, the horns as well as other regions of the housing are filled with a dielectric or oil coolant. Oil is commonly drawn through an output aperture located at one end of the housing, circulated through a radiator or heat exchanger and returned to an inlet aperture in the opposite end of the housing. The returned cooled fluid flows axially through the housing toward the outlet aperture, absorbing heat from the x-ray tube.
The x-ray tubes in CT scanners arc from time to time. The arcing changes the x-ray tube current, hence the x-ray output. Generally, the occurrence of arcing causes a CT scan to be discarded and retaken. In more extreme instances, the electronic circuitry that monitors the tube shuts it off. On restart, the tube functions normally again. As tubes age, the arcing normally becomes more frequent.